Air supply and pressure regulator devices are known, for instance for use in scuba diving, and generally have a double stage, where the first pressure reducing stage is connected to a high-pressure source of breathable gas, such as a tank filled to a pressure of 200-300 bar, and is designed to reduce said pressure to a predetermined intermediate pressure.
Generally, such first pressure reducing state includes a body having an inlet connected to a high-pressure source of breathable gas and an outlet for the breathable gas at a reduced pressure as compared with the inlet gas pressure. Said body is divided into at least one high-pressure gas chamber, communicating with said inlet, and a low-pressure gas chamber, connected with said outlet, said low-pressure gas chamber communicating with the high-pressure gas chamber by means of a pressure reducing valve.
Said pressure reducing valve comprises a valve seat that separates the high-pressure chamber from the low-pressure chamber, and cooperates with a shut-off member, having an enlarged head connected to a shaft, i.e. a piston shut-off member.
Such shut-off member is held within the high-pressure chamber and is axially displaceable, i.e. is parallel to its longitudinal axis, in either direction within said high-pressure chamber, such that the enlarged head alternately runs a stroke in which it moves off and away from the valve seat and a stroke in which it moves toward and against said valve seat. The shaft is supported in a fluid-tight manner in a guide hole formed in a wall of the high-pressure chamber, facing away from the valve seat, using a seal member, such as an O-ring, which is designed to delimit the opening of said hole.
Various types of shut-off members are known.
U.S. Pat. No. 5,139,046 discloses a metal piston shut-off member, having a head and a shaft integral with said head, with a molded coating of plastic material.
The plastic coating material extends on the body of the shut-off member from a groove formed in the head on the face facing away from the shaft to a shoulder formed near the lower end of the shaft. Therefore, the plastic coating covers all the parts of the shut-off member that are exposed to high pressure.
Particularly, as shown in FIG. 3, the plastic coating material is also provided in the shaft portion that is designed to cooperate with the seal member, i.e. the o-ring, that is adapted to delimit the guide hole through which the shaft of the shut-off member slides, which hole is formed in the wall of the high-pressure chamber, opposite to the valve seat. Thus, the shaft has no uncoated portions where the shut-off member is exposed to high pressure and the shaft and the plastic coating material have no junction surfaces in the high-pressure gas-containing part of the device, which might cause separation of the two materials: this will limit or avoid penetration of microbubbles between the shaft metal and the coating.
The shut-off member disclosed in U.S. Pat. No. 5,139,046 provides a simple process for securely molding plastic material, and avoids any junction between the metal and the plastic coating that might be exposed to high-pressure stress, thereby avoiding peeling damages or the like.
Nevertheless, since the plastic material coats both the head and the shaft of the shut-off member, such material shall be adapted to cooperate both with the valve seat and with the o-ring that delimits the opening through which the shaft slides, for sealing off the high-pressure chamber.
Therefore, the coating of the U.S. Pat. No. 5,139,046 has chemico-physical properties that result from a compromise between the rigidity required at the shaft, to allow the latter to slide through the opening, and the softness required at the head, to ensure sealing of the valve seat.
Particularly, if the coating material for the head and the shaft is too soft, it will be prone to deformation on the shaft where cooperation with the O-ring occurs, resulting in increased friction during the movement of the shut-off member and in delayed closing/opening of the valve seat. Conversely, if the coating material is too rigid, an imperfect seal may be obtained when the head of the shut-off member abuts against the valve seat.
U.S. Pat. No. 7,341,073 (Publication Number U.S. 2006/0097213) discloses a piston shut-off member having a central body at least partially covered with a first layer of rigid plastic material, such layer being at least partially covered with a second layer of plastic material, which is less rigid than the one of the first layer.
Like for U.S. Pat. No. 5,139,046, the first layer of rigid plastic material 3 extends substantially along the entire length of the shaft and particularly also covers the shaft portion that is designed to cooperate with the seal element, i.e. the O-ring, which is adapted to delimit the guide hole through which the shaft of the shut-off member slides. Therefore, the first layer of rigid plastic material 3 covers all the parts of the shut-off member that are exposed to high pressure, and avoid the presence of shaft/coating junction surfaces in the high-pressure area, that might cause peeling off of the coating, due to exposure to high-pressure gas.
In order to obviate the problem of plastic deformation on the shaft where cooperation with the O-ring occurs, resulting in increased friction during the movement of the shut-off member, as well as delayed closing/opening of the valve seat, and eventually inappropriate operation of the pressure regulator device, U.S. Pat. No. 7,341,073 provides a coating for the shut-off member that is composed of two plastic layers, where the head of the shut-off member is overcoated with a second layer of a plastic material that is softer than the first layer of plastic material that coats the shaft and head of the shut-off member.
Therefore, the rigid layer is more adapted to allow the shaft to slide through the O-ring that delimits the guide hole of the shut-off member, whereas the soft layer ensures optimal closing of the valve seat.
Nevertheless, this technical arrangement adds complexity to fabrication of the shut-off member, as it requires molding of two layers of chemically different materials, one above the other.
Particularly, since the shut-off member of U.S. Pat. No. 7,341,073 is required to be coated by two plastic coating layers that must be compatible with each other, this chemical compatibility between the rigid and less rigid layers will restrict the selectable materials.
Furthermore, in order to ensure that the shaft has no uncoated portion where the shut-off member is exposed to high pressure and to limit or prevent penetration of microbubbles between the shaft metal and the coating, the part of the shaft exposed to high pressure is coated as follows: as shown in FIG. 1 (where the shut-off member is shown in the valve seat closing position) and as described on page 3, lines 6-9, the first layer of the rigid plastic material (3) covers the shaft almost to the end opposite to the head, and particularly covers the shaft portion that is designed to cooperate with the seal element or O-ring, that delimits the guide and slide hole formed in the wall of the high-pressure chamber, which is opposite to the valve seat.
Such coating extends to such a length that said first coating layer contacts and cooperates with the O-ring in any operating state of the shut-off member, i.e. in either valve seat closing or opening state of said shut-off member. Therefore, the plastic layer that covers the shaft end opposite to the head, may be deformed or altered by contact with the seal element, which is designed to seal the high-pressure chamber at the hole through which said shaft end slides, resulting in increased friction during operation and/or gas leakage at said seal.
In further prior art technical arrangements, plastic overmolding is only provided at the head part of the shut-off member, using an adhesive layer to improve adhesion between the plastic layer (coated over a small area) and the metal that forms the body of the shut-off member, thereby limiting penetration of gas microbubbles between the layers of different natures.
The U.S. Pat. No. 5,746,198 discloses a metal shut-off member having a circular head supported by a shaft.
The shaft portion has no plastic coating, whereas the enlarged circular head is coated with a rubber-like plastic or elastomeric material.
Such plastic material is adhered to the metal head using chemical or mechanical means. This is required because the head of the shut-off member, namely the junction between the head and the shaft is exposed to high pressure and the plastic coating and the metal of the shut-off member are chemically incompatible.
An adhesive is provided between the head surface and the plastic or elastomeric material, to resist peeling off during exposure to high-pressure gas.
A shoulder is provided at the junction between the shaft and the head, whose diameter is slightly greater than the diameter of the shaft.
The shoulder provides support to a washer that is pushed against the shoulder (the washer is made of a metal that is preferably harder than the metal of the shoulder), to prevent peeling off of the plastic material during exposure to high pressure.
Nevertheless, these arrangements increase the complexity and costs for fabricating the shut-off member (as they require a combination of a metal washer and an adhesive layer for holding the plastic coating of the head in position) and do not prevent gas microbubbles in the high-pressure chamber from penetrating between the metal of the shut-off member and the plastic material, thereby causing the coating to be peeled off, and resulting in inadequate operation of the pressure regulator device.
A further prior art arrangement involves the use of a shut-off member having a rubber head, in which a rubber or elastomeric pad is embedded in a cup-shaped end housing formed on the head of the shut-off member. The drawback of this type of shut-off members is that rubber can be easily cut, and cause inadequate operation of the pressure reducing valve. Furthermore, due to the small rubber/metal contact area in the shut-off member, the rubber part may be easily peeled off, possibly due to penetration of gas microbubbles between the pad and the metal head.